Sheikh Abu Hamza, the London-based Islamic cleric and activist whose son is on trial in the Yemen, was arrested yesterday under the Prevention of Terrorism Act.

Two other men, including Yaser al-Sirri, an Egyptian asylum-seeker, were also arrested by 30 unarmed members of Scotland Yard's anti-terrorist branch in dawn raids in west London.

Scotland Yard, which declined to confirm the identity of the men, aged 36, 39, and 40, said the raids were part of a continuing investigation. They were arrested under section 14 of the Prevention of Terrorism Act which refers to the 'commission, preparation, or instigation of acts of terrorism'.

Under the act, people can be questioned for 48 hours and held for a further five days without being charged. The arrests came on the eve of Parliament's annual debate renewing the act.

Sheikh Hamza is head of the Supporters of Sharia, which he has described as a pressure group for oppressed Muslims. Haroon Rasheed, a spokesman for the group said: 'He is a scholar and he gives his Islamic views. He has only been arrested for questioning involving terrorist activities here. He has done no terrorist activities here.'

Akhtar Raja, lawyer for Mr Al-Sirri, last night described the arrests as a 'fishing expedition'.

Britain has been under pressure from Yemen to take action against Sheikh Hamza, with officials admitting that the Egyptian-born cleric had been under surveillance before the killings of British tourists and the arrests of Britons his son Mustafa one of them in Aden last December.

The Yemeni authorities have claimed Sheikh Hamza is linked to terrorist activities in their country. His son is among 10 British and French citizens being tried on charges of associating with armed groups, plotting murder and destruction, and possessing weapons. All 10 have been linked to the Islamic Army of Abyan, who kidnapped 16 Westerners in December, four of whom died in a shoot-out with Yemeni security forces Two weeks ago Hamza issued a statement in the name of the Islamic Army of Aden-Abyan, threatening the British and American ambassadors to Yemen.

Foreign Office sources have indicated that though extradition of Sheikh Abu Hamza as demanded by the Sana'a government is unlikely, prosecution was possible under controversial legislation passed last summer making it a criminal offence in Britain to conspire to commit terrorist acts abroad.

Whitehall sources say that the Supporters of Shariah is likely to be targeted to see if it throws up any information about worldwide Islamic fundamentalist groups, some of which are suspected to be loosely linked to the Afghan-based Saudi terrorist, Osama bin Ladin.

Egypt has been seeking Mr al-Sirri, though he denies Cairo's claim that he is a member of the banned Jihad group and that he was involved in the attempted assassination of an Egyptian prime minister. But British sources say Cairo has never produced evidence that would be admissable in a British court.

Mr al-Sirri runs the London-based Islamic Observer Centre, which he calls a 'registered charity defending human rights', and claims his movements are monitored by the security services.

Jack Straw, meanwhile, is in contact with the Egyptian authorities about the possibility of deporting Hany Yousseff, a prominent Egyptian defence lawyer who has been refused asylum here.

Khaled al-Fawwaz, a Saudi, is being held in Britain pending a hearing into a US request for his extradition allegedly in connection with the bombings of the US embassies in Kenya and Tanzania last August.

Britain had been under pressure from Egypt, Algeria, Bahrain, Tunisia, Saudi Arabia, Israel and France to move against exiled Muslim extremists well before last August's attacks. But the new legislation was seen as a sop to Washington.